Vitamin B12 binders (transcobalamins) in serum

Vitamin B12 inhibits peptidylarginine deiminases and ameliorates rheumatoid arthritis in CAIA mice

Rheumatoid arthritis is an autoimmune disease whose early onset correlates with dysregulated citrullination, a process catalyzed by peptidylarginine deiminase isoform 4 (PADI-4). Here, we report that PADI-4 is a novel target of vitamin B12, a water-soluble vitamin that serves as a cofactor in DNA synthesis and the metabolism of fatty and amino acids. Vitamin B12 preferentially inhibited PADI-4 over PADI-2 with comparable inhibitory activity to the reference compound Cl-amidine in enzymatic inhibition assays, and reduced total cellular citrullination levels including that of histone H3 citrullination mediated by PADI-4. We also demonstrated that hydroxocobalamin, a manufactured form of vitamin B12, significantly ameliorated the severity of collagen type II antibody induced arthritis (CAIA) in mice and diminished gene expression of the rheumatoid inflammatory factors and cytokines IL17A, TNFα, IL-6, COX-II and ANXA2, as well PADI-4. Therefore, the use of vitamin B12 to treat rheumatoid arthritis merits further study.

Macro-B12 and Unexpectedly High Levels of Plasma B12: A Critical Review

A low total plasma vitamin B12 supports a clinical suspicion of B12 deficiency, while the interpretation of an unexpectedly normal/high level is marred by controversies. Here, we critically review current knowledge on B12 in blood plasma, including the presence of the so-called "macro-B12". The latter form is most often defined as the fraction of B12 that can be removed by precipitation with polyethylene glycol (PEG), a nonspecific procedure that also removes protein polymers and antibody-bound analytes. Plasma B12 includes B12 attached to transcobalamin and haptocorrin, and an increased concentration of one or both proteins almost always causes an elevation of B12. The total plasma B12 is measured by automated competitive binding assays, often incorrectly referred to as immunoassays, since the binding protein is intrinsic factor and not an antibody. An unexpectedly high level of B12 may be further explored using immunological measurements of haptocorrin and transcobalamin (optionally combined with e.g., size-exclusion chromatography). Nonspecific methods, such as PEG precipitation, are likely to give misleading results and cannot be recommended. Currently, the need for evaluation of a high B12 of unknown etiology is limited since other tests (such as measurements of methylmalonic acid) may better guide the diagnosis of B12 deficiency.

Inborn errors of cobalamin absorption and metabolism

Derivatives of cobalamin (vitamin B(12)) are required for activity of two enzymes in humans. Adenosylcobalamin is required for activity of mitochondrial methylmalonylCoA mutase and methylcobalamin is required for activity of cytoplasmic methionine synthase. Deficiency in cobalamin, or inability to absorb cobalamin normally, can result in accumulation of methylmalonic acid and homocysteine in blood and urine. Methylmalonic acidemia can result in metabolic acidosis which in severe cases may be fatal. Hyperhomocysteinemia along with hypomethioninemia can result in hematologic (megaloblastic anemia, neutropenia, thrombocytopenia) and neurologic (subacute combined degeneration of the cord, dementia, psychosis) defects. Inborn errors affecting cobalamin absorption (inherited intrinsic factor deficiency, Imerslund–Gra¨ sbeck syndrome) and transport (transcobalamin deficiency) have been described. A series of inborn errors of intracellular cobalamin metabolism, designated cblA-cblG, have been differentiated by complementation analysis. These can give rise to isolated methylmalonic acidemia (cblA, cblB, cblD variant 2), isolated hyperhomocysteinemia (cblD variant 1, cblE, cblG) or combined methylmalonic acidemia and hyperhomocysteinemia (cblC, classic cblD, cblF). All these disorders are inherited as autosomal recessive traits. The genes underlying each of these disorders have been identified. Two other disorders, haptocorrin deficiency and transcobalamin receptor deficiency, have been described, but it is not clear that they have any consistent clinical phenotype.

Vitamin B12 and folate levels and lithium administration in patients with affective disorders

BACKGROUND

It is unclear whether there is a relationship between lithium administration and vitamin B12 metabolism.

METHODS

We compared serum B12, serum folate, and red blood cell folate concentrations in patients receiving and not receiving lithium at two Mood Disorders Clinics. As the two centers differed in vitamin assay methods, data were first analyzed separately and then combined. To rule out an in vitro effect of lithium on the assays, we also added varying amounts of lithium to lithium-free blood samples and measured vitamin concentrations.

RESULTS

Mean serum B12 concentrations were approximately 20% lower in the lithium than in the nonlithium group at each center. This difference was statistically significant for each center and on combination (two-tailed p = .017, .021, and .0009). The parametric effect size for each center and the combined weighted mean effect size were moderate in magnitude (.605, .523, and .565). There was a nonsignificant trend toward an increased prevalence of assay-defined B12 deficiency in the lithium group at one center only, with no cases in either group at the other center and a nonsignificant combined relative risk.

CONCLUSIONS

Our data may represent a lithium-associated decrease in serum B12 concentration. The clinical significance of these findings is not yet clear.

Cobalamin

Cobalamin (vitamin B12) is an essential nutrient derived exclusively from bacterial sources. It is an essential cofactor for three known enzymatic reactions. Untreated deficiency, caused by either the autoimmune disease pernicious anemia or nutritional lack, results in a macrocytic anemia and/or subacute combined degeneration of the spinal cord and is eventually fatal. Cobalamin in serum is bound to two proteins, transcobalamin and haptocorrin. The former is responsible for the essential delivery of cobalamin to most tissues. Inadequate tissue availability of cobalamin results in increased concentration of methylmalonic acid and homocyst(e)ine due to inhibition of methylmalonyl-CoA mutase and methionine synthase, respectively. Strict vegetarians have long been known to be at risk of cobalamin deficiency, which develops insidiously over many years. It is now clear that a significant number of the elderly and HIV-positive individuals are also at increased risk of deficiency. Any individual with reduced ability to split cobalamin from food-protein may also become deficient even though intrinsic factor is present. Diagnosis of cobalamin deficiency has frequently relied on total serum cobalamin and the Schilling test. Newer approaches such as analysis of methylmalonic acid, homocyst(e)ine, holotranscobalamin, anti-intrinsic factor antibodies, and serum gastrin may provide more cost-effective testing, as well as identify those with a covert deficiency.

Metabolite assays in cobalamin and folate deficiency

Cbl and folate are both necessary for the metabolism of HCYS, whereas only Cbl is required for MMA metabolism. During the past decade, analytical methods have been developed that are sensitive enough to detect low levels of MMA and HCYS normally present in the plasma. These methods are sufficiently precise to be used in the clinical laboratory and measurements of the serum levels of the metabolites provide sensitive and specific techniques for the identification of Cbl and folate deficiencies. These techniques constitute an important addition to the battery of diagnostic tests that are available for detecting the vitamin deficiencies and for distinguishing each from the other. By virtue of the role of Cbl and folate in the metabolic pathways that involve MMA and HCYS, levels of both metabolites rise in Cbl deficiency, but only HCYS rises in folate deficiency. During the development of Cbl or folate deficiencies, accumulation of these metabolites in the plasma signals the existence of a condition of biochemical vitamin deficiency of sufficient degree to cause impairment in the metabolic pathways which are dependent on these vitamins. Circulating metabolite levels appear to accurately reflect the nutritional status of the vitamins and a rise in serum metabolite levels is therefore one of the earliest and most reliable indicators of developing Cbl and folate deficiencies. Elevations of serum metabolites above the reference range not only precede a fall in the serum vitamin levels but also show a more consistent correlation with objective evidence of vitamin deficiency than do low blood vitamin levels. The advent of serum metabolite measurements has also made it possible to identify subtle or atypical forms of vitamin deficiency that may be associated with unusual or previously undiscovered disease manifestations. Thus, in patients who display only neurological manifestations of disease, underlying Cbl deficiency may be revealed by the finding of raised serum or urine levels of MMA. Similarly, unsuspected folate deficiency may be disclosed by the finding of a raised serum HCYS. This may have important implications with respect to disease risk, since there is mounting evidence that sub-optimal folate nutritional status may be associated with increased risks of vascular disease, neoplasia and birth defects. Finally, the measurement of serum levels of MMA, HCYS and other metabolites that accumulate in Cbl and folate deficiencies may provide important new insights into the mechanism whereby these vitamin deficiencies lead to different patterns and manifestations of disease.

Recent advances in elucidation of biological corrinoid functions

Eleven adenosylcorrinoid-dependent rearrangements and elimination reactions have been described during the last four decades of vitamin B12 research. In contrast, only the cobamide-dependent methionine synthase was well established as a corrinoid-dependent methyl transfer reaction. yet, investigations during the last few years revealed nine additional corrinoid-dependent methyltransferases. Many of these reactions are catalyzed by bacteria which possess a distinct C1 metabolism. Notably acetogenic and methanogenic bacteria carry out such methyl transfers in their anabolism and catabolism. Tetrahydrofolate or a similar pterine derivative is a key intermediate in these reactions. It functions as methyl acceptor and the methylated tetrahydrofolate serves as a methyl donor.

Inherited disorders of vitamin B12 metabolism

Inherited disorders of vitamin B12 include those which involve the inability of the vitamin to be absorbed from the gut and transported to the appropriate tissues, and those in which the vitamin is not utilised by target cells. The former include intrinsic factor abnormalities, selective malabsorption of vitamin B12 with proteinuria, and deficiencies of transcobalamin I and transcobalamin II. The latter include a defect in the release of free vitamin B12 from lysosomes (cblF), and defects in the formation of both vitamin B12 cofactors (cblC, cblD) or of adenosyl-B12 (cblA, cblB) or methyl-B12 alone (cblE, CblE variant). This article reviews the major clinical manifestations of these diseases, and provides an approach to the diagnosis of transcobalamin II deficiency and the cbl mutations using cultured cells.

Unsaturated Vitamin B12Binding Capacity in Human and Ruminant Blood Serum - A Comparison of Techniques Including a New Technique by High Performance Liquid Chromatography

A new technique by High Performance Liquid Chromatography (HPLC-gel permeation) shows promise as a tool to separate and quantitate the Unsaturated Vitamin B(12) Binding Capacity (UBSC) of the individual Vitamin B(12) binders in blood serum. This method, although not as rapid as protein-coated charcoal or cellulose separation techniques, is more applicable for use with large numbers of samples than gel filtration. The use of a radioactivity detector to monitor the eluant from the column permitted automation of the method. Comparable results for UBBC and for the UBBC of individual binders were obtained when samples were analyzed by gel filtration and HPLC. The HPLC method proved suitably precise and the recovery of added cyanocobalamin was acceptable. It is proposed that HPLC be the method of choice for measurement of the USBC of binders of Vitamin B(12) in blood serum.

Expression of transcobalamin II by amniocytes

Children with a genetic absence of transcobalamin 2 (TC2) are clinically asymptomatic at birth but develop severe megaloblastic anemia early in life. We have examined the incorporation of [57Co]-CN-B12 in the absence of any exogenous source of TC2 in control amniotic fluid derived cells and cultured diploid fibroblasts, and in fibroblasts from a patient with TC2 deficiency. Both control fibroblasts and amniocytes incorporated labelled B12 into TC2-B12, and the proportion of labelled TC2-B12 could be increased by growing cells in the presence of chloroquine which prevents intralysosomal hydrolysis of the TC2-B12 complex. In contrast, fibroblasts from the patient with TC2 deficiency incorporated almost no label as TC2-B12. These studies suggest that TC2 deficiency either due to aberrant production of TC2 or because of the production of an abnormal TC2 which does not bind B12 can be diagnosed before birth.

A patient with the inability to maintain in vivo levels of bound cobalamin (Cbl) and manifestations of tissue deficiency of Cbl

A 39-year-old woman presented with mild anemia, glossitis, an increased MCV, a low serum cobalamin (Cbl) (vitamin B12), mild tissue deficiency of Cbl, but with neither malabsorption of Cbl, impaired intake, nor deficiency of or inactivity of transcobalamin II (TC II). Because of a persistently low holo-TC II (TC II carrying Cbl as the circulating complex of TC II-Cbl), much of the evaluation was focused on the patient's TC II. Her TC II promoted the uptake of Cbl, reacted with anti-TC II, and bound Cbl in vitro. A test dose of 200 micrograms of cyanocobalamin (CN-Cbl) i.m. increased her holo TC II to levels higher than those in healthy persons, but with a much more abrupt fall to a subnormal level. Two milligrams of CN-Cbl i.m. followed by 100 micrograms i.m. monthly failed to maintain normal amounts of circulating TC II-Cbl or to overcome the tissue deficiency of Cbl. One milligram i.m. weekly or daily p.o. corrected both. The low holo TC II was considered to be responsible for the clinical expression and may have been primary to the reduced amounts of total and holo R binder of Cbl in the circulation. This study of a newly recognized defect points out the need for circulating holo TC II, a rational use of pharmacologic amounts of Cbl, and a possible interrelationship between TC II and the R binder of Cbl.